The present invention relates to gas turbine engines, and more particularly, to an epicyclic gear system for use in gas turbine engines.
Epicyclic gear trains are complex mechanisms for reducing or increasing the rotational speed between two rotating shafts or rotors. The compactness of planetary or star system gear trains makes them appealing for use in aircraft engines.
The forces and torque transferred through an epicyclic gear train place tremendous stresses on the gear train components, making them susceptible to breakage and wear. For example, the longitudinal axes of an epicyclic gear train's sun gear, star gear, and ring gear are ideally parallel with the longitudinal axis of an external shaft that rotates the sun gear. Unfortunately, many components in the epicyclic gear train are generally not effectively aligned due to numerous factors including imbalances in rotating hardware, manufacturing imperfections, and transient flexure of shafts and support frames due to aircraft maneuvers. Due to the rarity of coaxial alignment, each star gear transfers varying amounts of torque and loads from the sun gear to the ring gear. To account for these varying amounts of torque on the star gears, one must design each star gear and its corresponding journal bearing (which mounts the star gear to a gear carrier) to carry greater torque than the average torque on all the star gears/journal bearings combined. Typically, each star gear and its corresponding journal bearing are designed to withstand a torque of about 37 percent higher than the mean torque across all the star gears. Unfortunately, by designing each journal bearing and star gear to withstand torque about 37 percent higher than the mean torque one must sacrifice epicyclic gear system size and weight. This leads to greater epicyclic gear system size and weight in the aircraft engine. Additionally, precise journal bearing and star gear tolerances are required to achieve the 37 percent design factor, increasing overall manufacturing cost of the epicyclic gear system.